My research interests focus on examining several variants of the MPC 11 IgG2b-producing mouse myeloma cell lines which synthesize altered immunoglobulin heavy chains. Some altered heavy chains are shorter than the parent, having molecular weights of 50,000 or 40,000 compared to the parental size of 55,000; others have serological, peptide and assembly characteristics of a new subclass, IgG2a. The latter are especially interesting because they represent the expression of a previously silent constant region gene. We are studying the structural relationships of these variants to each other, to the parental MPC 11, and to MOPC 173, an IgG2a protein of known sequence. We have shown that the several IgG2a variant proteins retain the parental idiotype, yet differ from each other by peptide maps, charge and assembly parameters. These same parameters have enabled us to subgroup the variants. The Fc of one variant protein is entirely gamma 2a-like while the Fc of a second is a gamma 2b-gamma 2a hybrid. We hope that the continued structural analysis of these variants will give us a clue to the mechanisms that control the expression and arrangement of immunoglobulin genes.